I use Whirlfloc tablets (a mixture of irish moss and some other ingredient I'm not sure of) all the time brewing extracts and have never had a problem. You can definitely tell a difference in the clarity when using it and it has no effect on the taste.
If you are going to do a secondary, I would leave it in the fermenter for at least 7 days. I don't do secondarys anymore and just leave it in the fermenter for 3 weeks. The less I mess with it the better it comes out, but to each his own.
Im going to be brewing a higher gravity beer so I bought a larger bucket with more headspace just to be safe in case the yeast decide to go crazy. I don't transfer to secondary and just leave beer in the primary(about a month) until im ready to bottle. Do I have to worry about the extra...